The metal dialkyl- and diaryl-dithiophosphates, particularly those containing zinc, have been widely used for many years to protect metal surfaces under friction against wear and corrosion, as well as to protect lubricating oils containing the same from oxidation. Details of manufacture of such additives are disclosed for example in U.S. Pat. Nos. 2,364,283--2,364,284,--2,365,938,--2,410,650,--2,438,876 and 3,190,833. The metal dialkyl- and dialkylaryl-dithiophosphates disclosed in the prior art are generally characterized by Metal/P/S atomic proportions of 1/2/4--although the sulfur proportion may be higher as the result of a further sulfurization, as described for example in U.S. Pat. No. 2,438,876. An efficient protection against wear requires the use of such products in lubricating oils in proportions by weight generally ranging from 0.5 to 1.5%. On the other hand, German patent No. 948,156 teaches that it is possible to obtain thiolthionophosphoric acid derivatives by reacting a thiolthionophosphoric (or dithiophosphoric) acid diester with ethylene oxide to form a triester of thiolthionophosphoric acid with an oxygenated group, and then treating said product with phosphorus pentasulfide. Although these derivatives are indicated as being liable to form salts with various metals (alkali, alkaline-earth, heavy metals) or with amines, only the preparation of an oleylamine salt is effectively disclosed.